The present invention relates to a printed circuit board design support apparatus, method, and program and, more particularly, to a printed circuit board design support apparatus, method, and program which calculate the radiation amount of electromagnetic radiation from interconnections of a printed circuit board on the basis of printed circuit board design information related to the printed circuit board, components, and interconnections, thereby supporting design of a printed circuit board whose unwanted electromagnetic radiation is suppressed.
To prevent any adverse effect of unwanted electromagnetic waves radiated from electronic devices on public broadcastings or communications, it is necessary to suppress unwanted electromagnetic radiation from electronic devices and printed circuit boards mounted on the electronic devices. However, it is difficult to suppress electromagnetic radiation because causes thereof are hard to find. In some cases, to suppress electromagnetic radiation, a ferrite core is attached to the proximal end of the cable of a device, or a ferrite bead, a damping resistor, or various kinds of filters are inserted to a high-speed signal interconnection of a printed circuit board, resulting in an increase in cost of products. If even such measures are insufficient, boards are sometimes re-designed or re-manufactured. This delays shipping.
To suppress electromagnetic radiation without increasing the cost or delaying shipping, it is preferable to use a design method of suppressing electromagnetic radiation at the time of designing a printed circuit board. This is because the cost for correction can be suppressed at an upstream stage of design and development.
Under these circumstances, apparatuses for supporting design of a printed circuit board whose unwanted electromagnetic radiation is suppressed, design support methods, and storage media which store programs for supporting design have been conventionally proposed. Examples are xe2x80x9cPrinted Board CAD Apparatusxe2x80x9d disclosed in Japanese Patent Laid-Open No. 5-67176, xe2x80x9cCircuit Board Design Method and Storage Mediumxe2x80x9d disclosed in Japanese Patent Laid-Open No. 10-49568, and xe2x80x9cDesign Support Apparatusxe2x80x9d disclosed in Japanese Patent Laid-Open No. 2001-134626.
FIG. 22 shows an embodiment of xe2x80x9cPrinted Board CAD Apparatusxe2x80x9d disclosed in Japanese Patent Laid-Open No. 5-67176. In this prior art, a board surface is divided into a mesh pattern, and the current value of a signal path included in each divided mesh element is calculated. The visual attribute of the mesh is determined in accordance with the magnitude of the current value or a radiation noise predicted value for each mesh element, which is calculated on the basis of the current value. According to this prior art, the designer of a printed circuit board can intuitively understand the magnitude of the radiation noise amount on the basis of the magnitude of the current value or radiation noise predicted value indicated on the matrix. In addition, when the designer determines interconnections so as not to locally increase the indicator level, the noise amount on the board can be naturally dispersed.
FIG. 23 shows a flow chart of xe2x80x9cCircuit Board Design Method and Storage Mediumxe2x80x9d disclosed in Japanese Patent Laid-Open No. 10-49568. In this prior art, in steps S22 to S24, signal lines are virtually interconnected on a virtual section read out from a virtual section description table, and an unwanted radiation amount X is calculated. In step S27, for a signal interconnection whose unwanted radiation amount X exceeds an allowable value A, an improved solution N1 for an improved virtual section and an improved solution N2 with a target component inserted are calculated. In step S28, the improved solutions N1 and N2 are assigned to layer structures read out from a layer structure description table. In steps S29 and S30, practical solutions P are extracted from combinations of the improved solutions N1 and N2 and the layer structures. An optimum solution Q is selected from the practical solutions P on the basis of the manufacturing cost and unwanted radiation amount. In step S31, the signal lines are automatically interconnected to a layer structure determined by the optimum solution Q. By this prior art, radiation noise can be evaluated and noise measures can be taken at an early stage of board design.
FIG. 24 shows the overall arrangement of xe2x80x9cDesign Support Apparatusxe2x80x9d disclosed in Japanese Patent Laid-Open No. 2001-134626. As a characteristic feature, this prior art comprises a means (design data storage means 107) for storing design information related to boards, components, and networks, a means (layout component selection means 110) for selecting a component to be laid out, a means (high-speed network search means 111) for searching the design data storage means for a high-speed network to be connected to the component to be laid out, a means (virtual interconnection path determination means 940) for determining a virtual interconnection path of the high-speed network searched by the high-speed network search means, a means (radiation noise calculation means 950) for calculating radiation noise of the high-speed network interconnected to the virtual interconnection path, and a means (radiation noise indicating information generation means 115) for generating information that indicates radiation noise. According to this prior art, radiation noise simulation is done in laying out a component, and a radiation noise amount for the high-speed network connected to the component (layout component) to be laid out can be indicated to the designer in various indicating patterns. For this reason, an appropriate layout position of each layout component can be obtained, and interconnection design with a satisfactory radiation noise characteristic can be done.
In the above prior-art methods, as electromagnetic radiation from a printed circuit board, only electromagnetic radiation from signal interconnections (electromagnetic radiation called differential mode radiation or normal mode radiation) is handled.
In differential mode radiation, a closed circuit formed by signal interconnections acts like a loop antenna and emits electromagnetic radiation. As a general example, when a transmitting-side IC and receiving-side IC are connected by a microstrip line formed from an interconnection pattern and ground plane, a current flowing through the interconnection pattern and a mirror-image current of the interconnection pattern centered about the ground plane flow as a current of the closed loop. This causes radiation as in a loop antenna. Details are described in, e.g., IEICE TRAN. COMMUN., VOL. E78-B, NO. 2 February 1995, xe2x80x9cPrediction of Peak Frequencies on Electromagnetic Emission from a Signal Line on a Printed Circuit Boardxe2x80x9d.
Electromagnetic radiation from a printed circuit board includes not only the radiation from a signal interconnection but also radiation from a ground plane opposing a signal interconnection (electromagnetic radiation called common mode radiation or asymmetrical mode radiation). For some boards, the common mode radiation is more dominant, as is reported. For example, reference 1 (R. Dockey, xe2x80x9cAsymmetrical Mode Radiation from Multi-layer Printed Circuit Boardsxe2x80x9d EMC/ESD International Symposium, 1992, pp. 247-251) reports that a ground plane acts like a dipole antenna due to a current flowing through a signal interconnection, the entire ground plane causes resonance to induce strong electromagnetic radiation, and the radiation amount changes depending on the width of the ground plane.
Reference 2 (B. Archambeault, xe2x80x9cModeling of EMI Emissions from Microstrip Structures with Imperfect Reference Planesxe2x80x9d, IEEE International Symposium on Electromagnetic Compatibility, Austin, 1997, pp. 456-461) reports that as the distance between a signal interconnection and an edge portion of a ground plane decreases, the electromagnetic radiation amount from a printed circuit board increases. These reports suggest that to design a printed circuit board with suppressed electromagnetic radiation, common mode radiation from a ground plane whose radiation amount largely changes depending on the size of the ground plane or the position of the signal interconnection should be taken into consideration as well as differential mode radiation from a signal interconnection.
Hence, in calculating an electromagnetic radiation amount from a signal interconnection, to calculate the radiation amount of common mode radiation in addition to the radiation amount of differential mode radiation obtained by a conventional method, a method using electromagnetic field simulation as in reference 2 can be used.
However, to calculate the radiation amount of a large-scale printed circuit board having a number of signal interconnections using such an electromagnetic field simulator, a large memory must be prepared. In addition, it takes a long calculation time. For these reasons, it is very difficult in the present circumstances to execute electromagnetic field simulation simultaneously with board design.
The present invention has been made to solve the above problem, and has as its object to provide a printed circuit board design support apparatus, method, and program which are capable of easily calculating an amount of electromagnetic radiation that occurs due to an interconnection of a printed circuit board and easily and accurately grasping the electromagnetic radiation amount of each interconnection without executing electromagnetic field simulation.
In order to achieve the above object, according to an aspect of the present invention, there is provided a printed circuit board design support apparatus for supporting design of a printed circuit board by calculating a radiation amount of electromagnetic radiation caused by an interconnection on the basis of design information related to each of the printed circuit board having a ground plane, interconnections formed on the printed circuit board, and components to be mounted on the printed circuit board, comprising arithmetic means for calculating a common mode (CM) radiation amount of the interconnection on the basis of a CM radiation amount ratio that indicates a ratio of a common mode (CM) radiation amount of electromagnetic radiation caused by the ground plane in correspondence with the interconnection to a differential mode (DM) radiation amount of electromagnetic radiation caused by the interconnection.
The arithmetic means may calculate, as a major (MAJ) radiation amount that indicates a major radiation amount of electromagnetic radiation caused by the interconnection, a radiation amount corresponding to a sum of the differential mode (DM) radiation amount and common mode (CM) radiation amount of the interconnection. For example, the arithmetic means may calculate the major (MAJ) radiation amount of the interconnection on the basis of an MAJ radiation amount ratio that indicates a ratio of the major (MAJ) radiation amount to the differential mode (DM) radiation amount of the interconnection.
When a radiation amount from each interconnection is to be evaluated, the apparatus may further comprise comparison means for comparing the major (MAJ) radiation amount of the interconnection, which is calculated by the arithmetic means, with a limit value that indicates a desired limit of the radiation amount of the interconnection and outputting a comparison result.
As a detailed arrangement for calculating the common mode (CM) radiation amount of the interconnection, the arithmetic means may comprise DM radiation amount calculation means for calculating the differential mode (DM) radiation amount of the interconnection on the basis of the design information, CM radiation amount ratio calculation means for calculating the CM radiation amount ratio corresponding to a size of the printed circuit board and a position of the interconnection on the printed circuit board on the basis of a relationship, obtained in advance, between the CM radiation amount ratio and the size of the printed circuit board and the position of the interconnection on the printed circuit board, and CM radiation amount calculation means for calculating the common mode (CM) radiation amount of the interconnection on the basis of the CM radiation amount ratio calculated by the CM radiation amount ratio calculation means and the differential mode (DM) radiation amount calculated by the DM radiation amount calculation means.
As a detailed arrangement for calculating the major (MAJ) radiation amount of the interconnection, the arithmetic means may comprise DM radiation amount calculation means for calculating the differential mode (DM) radiation amount of the interconnection on the basis of the design information, CM radiation amount ratio calculation means for calculating the CM radiation amount ratio corresponding to a size of the printed circuit board and a position of the interconnection on the printed circuit board on the basis of a relationship, obtained in advance, between the CM radiation amount ratio and the size of the printed circuit board and the position of the interconnection on the printed circuit board, MAJ radiation amount ratio calculation means for calculating an MAJ radiation amount ratio that indicates a ratio of the major (MAJ) radiation amount to the differential mode (DM) radiation amount of the interconnection on the basis of the CM radiation amount ratio calculated by the CM radiation amount ratio calculation means, and MAJ radiation amount calculation means for calculating the major (MAJ) radiation amount of the interconnection on the basis of the MAJ radiation amount ratio calculated by the MAJ radiation amount ratio calculation means and the differential mode (DM) radiation amount calculated by the DM radiation amount calculation means.
To calculate the common mode (CM) radiation amount of the interconnection, the arithmetic means may further comprise CM radiation amount calculation means for calculating the common mode (CM) radiation amount of the interconnection on the basis of the CM radiation amount ratio calculated by the CM radiation amount ratio calculation means and the differential mode (DM) radiation amount calculated by the DM radiation amount calculation means.
As a detailed arrangement for calculating the major (MAJ) radiation amount of the interconnection, the arithmetic means may comprise DM radiation amount calculation means for calculating the differential mode (DM) radiation amount of the interconnection on the basis of the design information, CM radiation amount ratio calculation means for, letting a be a width of the ground plane opposing the interconnection in a direction parallel to the interconnection, b be a width in a direction perpendicular to the interconnection, and d be a distance from an edge of the ground plane to the interconnection, calculating the CM radiation amount ratio CM/DM(a,b,d) that indicates the ratio of the common mode (CM) radiation amount to the differential mode (DM) radiation amount of the interconnection by                     CM        /        DM            ⁢              (                  a          ,          b          ,          d                )              =          10                                                  y              0                        ⁡                          (                              a                ,                b                            )                                +                      0.35            ·                          exp              [                                                                    1                    -                    d                                                        b                    /                    2                                                                    t                  ⁡                                      (                                          a                      ,                      b                                        )                                                              ]                                      20                                y        0            ⁡              (                  a          ,          b                )              =          0.57      +              24.47        ·                  log          ⁡                      (                          a              b                        )                              -              3.83        ·                              [                          log              ⁡                              (                                  a                  b                                )                                      ]                    2                                        t        ⁡                  (                      a            ,            b                    )                    =              0.26        +                  0.01          ·                      (                          a              b                        )                                ,  
MAJ radiation amount ratio calculation means for calculating an MAJ radiation amount ratio MAJ/DM(a,b,d) that indicates a ratio of the major (MAJ) radiation amount to the differential mode (DM) radiation amount of the interconnection on the basis of the CM radiation amount ratio CM/DM(a,b,d) calculated by the CM radiation amount ratio calculation means by
MAJ/DM(a,b,d)=1+CM/DM(a,b,d), 
CM radiation amount calculation means for calculating the common mode (CM) radiation amount of the interconnection on the basis of the CM radiation amount ratio calculated by the CM radiation amount ratio calculation means and the differential mode (DM) radiation amount calculated by the DM radiation amount calculation means, and MAJ radiation amount calculation means for calculating the major (MAJ) radiation amount of the interconnection on the basis of the MAJ radiation amount ratio calculated by the MAJ radiation amount ratio calculation means and the differential mode (DM) radiation amount calculated by the DM radiation amount calculation means.
When each radiation amount is to be calculated particularly for each interconnection element of the interconnection, the DM radiation amount calculation means may calculate the differential mode (DM) radiation amount of the entire interconnection by calculating a differential mode (DM) radiation amount EDM(L) letting L be a length of each of a plurality of interconnection elements obtained by dividing the interconnection along two directions perpendicular to each other, and adding the differential mode (DM) radiation amounts EDM(L), the CM radiation amount ratio calculation means may calculate the CM radiation amount ratio CM/DM(a,b,d) for each interconnection element, the MAJ radiation amount ratio calculation means may calculate the MAJ radiation amount ratio MAJ/DM(a,b,d) for each interconnection element, the CM radiation amount calculation means may calculate the common mode (CM) radiation amount of the entire interconnection on the basis of the differential mode (DM) radiation amount EDM(L) and MAJ radiation amount ratio MAJ/DM(a,b,d) of each interconnection element by             E      CM        =          ∑                        CM          /                      DM            ⁢                          (                              a                ,                b                ,                d                            )                                      ·                              E            DM                    ⁢                      (            L            )                                ,
and the MAJ radiation amount calculation means may calculate the major (MAJ) radiation amount of the entire interconnection on the basis of the differential mode (DM) radiation amount EDM(L) and MAJ radiation amount ratio MAJ/DM(a,b,d) of each interconnection element by       E    MAJ    =      ∑                  MAJ        /                  DM          ⁢                      (                          a              ,              b              ,              d                        )                              ·                        E          DM                ⁢                  (          L          )                    
According to another aspect of the present invention, there is provided a printed circuit board design support apparatus for supporting design of a printed circuit board by calculating a radiation amount of electromagnetic radiation caused by an interconnection on the basis of design information related to each of the printed circuit board having a ground plane, interconnections formed on the printed circuit board, and components to be mounted on the printed circuit board, comprising arithmetic means for calculating, as a major (MAJ) radiation amount that indicates a major radiation amount of electromagnetic radiation caused by the interconnection, a radiation amount corresponding to a sum of a differential mode (DM) radiation amount of electromagnetic radiation caused by the interconnection and a common mode (CM) radiation amount of electromagnetic radiation caused by the ground plane in correspondence with the interconnection.
As a detailed arrangement for calculating the major (MAJ) radiation amount of the interconnection, the arithmetic means may comprise DM radiation amount calculation means for calculating the differential mode (DM) radiation amount of the interconnection on the basis of the design information, CM radiation amount ratio calculation means for calculating a CM radiation amount ratio that indicates a ratio of the common mode (CM) radiation amount to the differential mode (DM) radiation amount of the interconnection on the basis of the design information, MAJ radiation amount ratio calculation means for calculating an MAJ radiation amount ratio that indicates a ratio of the major (MAJ) radiation amount to the differential mode (DM) radiation amount of the interconnection on the basis of the CM radiation amount ratio calculated by the CM radiation amount ratio calculation means, CM radiation amount calculation means for calculating the common mode (CM) radiation amount of the interconnection on the basis of the CM radiation amount ratio calculated by the CM radiation amount ratio calculation means and the differential mode (DM) radiation amount calculated by the DM radiation amount calculation means, and MAJ radiation amount calculation means for calculating the major (MAJ) radiation amount of the interconnection on the basis of the MAJ radiation amount ratio calculated by the MAJ radiation amount ratio calculation means and the differential mode (DM) radiation amount calculated by the DM radiation amount calculation means.
According to an aspect of the present invention, there is provided a printed circuit board design support method used in a printed circuit board design support apparatus for supporting design of a printed circuit board by calculating a radiation amount of electromagnetic radiation caused by an interconnection on the basis of design information related to each of the printed circuit board having a ground plane, interconnections formed on the printed circuit board, and components to be mounted on the printed circuit board, comprising causing arithmetic means of the printed circuit board design support apparatus to calculate a common mode (CM) radiation amount of the interconnection on the basis of a CM radiation amount ratio that indicates a ratio of a common mode (CM) radiation amount of electromagnetic radiation caused by the ground plane in correspondence with the interconnection to a differential mode (DM) radiation amount of electromagnetic radiation caused by the interconnection.
The arithmetic means may calculate, as a major (MAJ) radiation amount that indicates a major radiation amount of electromagnetic radiation caused by the interconnection, a radiation amount corresponding to a sum of the differential mode (DM) radiation amount and common mode (CM) radiation amount of the interconnection.
As a detailed procedure, the arithmetic means may calculate the differential mode (DM) radiation amount of the interconnection on the basis of the design information, the CM radiation amount ratio corresponding to a size of the printed circuit board and a position of the interconnection on the printed circuit board on the basis of a relationship, obtained in advance, between the CM radiation amount ratio and the size of the printed circuit board and the position of the interconnection on the printed circuit board, the common mode (CM) radiation amount of the interconnection on the basis of the CM radiation amount ratio and differential mode (DM) radiation amount, an MAJ radiation amount ratio that indicates a ratio of the major (MAJ) radiation amount to the differential mode (DM) radiation amount of the interconnection on the basis of the CM radiation amount ratio, and the major (MAJ) radiation amount of the interconnection on the basis of the MAJ radiation amount ratio and differential mode (DM) radiation amount.
As a more detailed procedure, the arithmetic means may calculate the differential mode (DM) radiation amount of the interconnection on the basis of the design information, letting a be a width of the ground plane opposing the interconnection in a direction parallel to the interconnection, b be a width in a direction perpendicular to the interconnection, and d be a distance from an edge of the ground plane to the interconnection, the CM radiation amount ratio CM/DM(a,b,d) that indicates the ratio of the common mode (CM) radiation amount to the differential mode (DM) radiation amount of the interconnection by                     CM        /        DM            ⁢              (                  a          ,          b          ,          d                )              =          10                                                  y              0                        ⁡                          (                              a                ,                b                            )                                +                      0.35            ·                          exp              [                                                                    1                    -                    d                                                        b                    /                    2                                                                    t                  ⁡                                      (                                          a                      ,                      b                                        )                                                              ]                                      20                                y        0            ⁡              (                  a          ,          b                )              =          0.57      +              24.47        ·                  log          ⁡                      (                          a              b                        )                              -              3.83        ·                              [                          log              ⁡                              (                                  a                  b                                )                                      ]                    2                                        t        ⁡                  (                      a            ,            b                    )                    =              0.26        +                  0.01          ·                      (                          a              b                        )                                ,  
an MAJ radiation amount ratio MAJ/DM(a,b,d) that indicates a ratio of the major (MAJ) radiation amount to the differential mode (DM) radiation amount of the interconnection on the basis of the CM radiation amount ratio CM/DM(a,b,d) by
MAJ/DM(a,b,d)=1+CM/DM(a,b,d), 
the common mode (CM) radiation amount of the interconnection on the basis of the CM radiation amount ratio and differential mode (DM) radiation amount, and the major (MAJ) radiation amount of the interconnection on the basis of the MAJ radiation amount ratio and differential mode (DM) radiation amount.
According to another aspect of the present invention, there is provided a printed circuit board design support method used in a printed circuit board design support apparatus for supporting design of a printed circuit board by calculating a radiation amount of electromagnetic radiation caused by an interconnection on the basis of design information related to each of the printed circuit board having a ground plane, interconnections formed on the printed circuit board, and components to be mounted on the printed circuit board, comprising causing arithmetic means of the printed circuit board design support apparatus to calculate, as a major (MAJ) radiation amount that indicates a major radiation amount of electromagnetic radiation caused by the interconnection, a radiation amount corresponding to a sum of a differential mode (DM) radiation amount of electromagnetic radiation caused by the interconnection and a common mode (CM) radiation amount of electromagnetic radiation caused by the ground plane in correspondence with the interconnection.
A program according to the present invention causes a computer arranged in a printed circuit board design support apparatus to execute the above-described procedures of the printed circuit board design support method. The steps of the program are the same as the above-described procedures of the printed circuit board design support method according to the present invention, and a description thereof will be omitted.
In the present invention having the above arrangement, the above-described problem is solved by the following functions.
First, that electromagnetic radiation from a printed circuit board corresponds to the synthesized value of differential mode (DM) radiation from a signal interconnection and common mode (CM) radiation from a ground plane will be described with reference to the accompanying drawings.
FIG. 13 shows a printed circuit board. A board 30 is a four-layered printed circuit board having a signal layerxe2x80x94ground layerxe2x80x94power supply layerxe2x80x94signal layer. The size of the board 30 is 210 mm (length)xc3x97100 mm (width)xc3x971.6 mm (thickness). A quartz oscillator 31, LSI 32, load capacitors 33, and signal interconnections 34 that connect these components are arranged on the first layer serving as a signal layer. A 40-MHz clock signal is input from the quartz oscillator 31 to the LSI 32. A 20-MHz rectangular wave signal is output from the LSI 32 to the 16 7-pF (pico-farad) load capacitors 33. In addition, an initializing circuit 35 is arranged in a region adjacent to the LSI 32 on the first layer serving as a signal layer.
Since the 16 signal interconnections 34 arranged in parallel simultaneously operate, radiation from the board 30 is supposed to correspond to a synthesized value of radiation from the signal interconnections 34 and radiation from the ground plane (not shown) of the layer in the board 30. Within the range of 30 MHz to 1 GHz that comes into question for unwanted electromagnetic radiation, the length of the signal interconnection 34 is about 3 cm corresponding to a {fraction (1/10)} wavelength of 1 GHz. Hence, the signal interconnections 34 act as a microloop antenna. Radiation from this microloop antenna is DM radiation.
Since the ground plane is excited in a direction along the signal interconnections, the long side of the ground plane acts as a dipole antenna. Radiation by this dipole antenna is CM radiation. When radiation amounts from the two antennas are synthesized, maximum radiation from the printed circuit board is determined.
FIGS. 14 to 16 show measurement results of radiation patterns of the board. The results were obtained by measuring field strength radiation patterns while arranging the board in a fully anechoic room with radio wave absorbers even on the floor surface. The distance between the board and a measurement antenna was 3 m. The height of the substrate and that of the measurement antenna were 1.5 m each. Measurements were executed while rotating the board through 360xc2x0 in a plane parallel to the floor surface. FIG. 14 shows a result obtained when the board surface was arranged in parallel to the floor surface. FIG. 15 shows a result obtained when the board long side was arranged to be perpendicular to the floor surface. FIG. 16 shows a result obtained when the board short side was arranged to be perpendicular to the floor surface. FIGS. 14 to 16 also show the conceptual views of the radiation patterns of DM radiation and CM radiation. These results were obtained by measuring a 520-MHz component with the highest radiation level. A solid line indicates a horizontally polarized wave component. A broken line indicates a vertically polarized wave component. Referring to FIG. 14, the major polarized wave of DM radiation is a vertically polarized wave, and the major polarized wave of CM radiation is a horizontally polarized wave. As for the measurement results, both the shapes and the polarized waves match the conceptual views of DM radiation and CM radiation. For this reason, the maximum value of the horizontally polarized wave and that of the vertically polarized wave on the observation surface indicate the maximum value of CM radiation and the maximum value of DM radiation, respectively.
Referring to FIG. 15, both the major polarized wave of DM radiation and that of CM radiation are vertically polarized waves. Hence, in the measurement results as well, the major polarized wave is a vertically polarized wave whose level is maximized when xcex8=0xc2x0 because the maximum value of DM radiation and that of CM radiation are synthesized. Referring to FIG. 16, both the major polarized wave of DM radiation and that of CM radiation are horizontally polarized waves. Hence, in the measurement results as well, the major polarized wave is a horizontally polarized wave whose level is maximized when xcfx86=0xc2x0 because the maximum value of DM radiation and that of CM radiation are synthesized.
The level of the electromagnetic wave (30 MHz to 1 GHz) measured here rarely abruptly rises at a specific position, unlike light having a shorter wavelength. For this reason, the maximum radiation level from the board is thoroughly observed by observing the three observation surfaces perpendicular to each other. The vertically polarized wave (Excfx86) on the observation surface xcex8=0xc2x0 in FIG. 15 and the horizontally polarized wave (Excfx86) on the observation surface xcfx86=0xc2x0 in FIG. 16 are polarized waves at the same position. In FIGS. 15 and 16, the highest radiation level is observed at this position. Hence, it indicates maximum radiation from the board.
That is, the vertically polarized wave and horizontally polarized wave shown in FIG. 14 indicate DM radiation and CM radiation, respectively. The vertically polarized wave shown in FIG. 15 and the horizontally polarized wave shown in FIG. 16 indicate a synthesized value of DM radiation and CM radiation. In addition, the maximum value on the observation surface where DM radiation and CM radiation are synthesized corresponds to maximum radiation from the printed circuit board.
The present inventor found from the above examinations that when sufficiently short signal interconnections were handled, DM radiation, CM radiation, and maximum radiation as a synthesized value of DM radiation and CM radiation, i.e., a major (MAJ) radiation amount that indicates the major radiation amount of electromagnetic radiation caused by the signal interconnections could be separated by the polarizing characteristic of electromagnetic radiation from the printed circuit board. On the basis of this finding, FIGS. 17, 18, and 19 show the ratio (CM/DM) of a CM radiation amount to a DM radiation and the ratio (MAJ/DM) of a major radiation amount to a DM radiation amount. As for the measurement conditions, FIGS. 14, 15, and 16 correspond to FIGS. 17, 18, and 19, respectively.
Referring to FIGS. 17, 18, and 19, ▪ indicates a result obtained when a coefficient was obtained from radiation field measurement result. A solid line indicates a result obtained when a coefficient was obtained by a commercially available FDTD (Finite Difference Time Domain) electromagnetic field simulator.
In the electromagnetic field simulation, the ground plane of a printed circuit board was modeled as a perfect conductive plate having the same planar size and an infinitesimal thickness. An interconnection portion for connecting an LSI and a load capacitor was modeled as a perfect conductive rod having an infinitesimal thickness. The semiconductor chip of an LSI was modeled as a voltage source. A load capacitor was modeled as a 1-xcexa9 resistance component. The remaining components such as a thin dielectric member serving as a board material were neglected. As a simulation condition, a 10-layered PML (Perfect Matched Layer) was used for an absorption boundary condition. A Gaussian pulse was used as an input pulse. Under these conditions, radiation patterns were calculated within the range of 200 MHz to 1 GHz at a pitch of 20 MHz, a DM radiation component, CM radiation component, and a maximum radiation component corresponding to the sum of the DM and CM radiation components were extracted from the polarizing characteristic, thereby obtaining the ratios CM/DM and MAJ/DM. Since the envelopes of these ratios satisfactorily match in a broad band, the above finding can be applied in a broad band.
A DM radiation amount is determined by the shape of a signal interconnection or the value of a flowing current and therefore does not depend on the position of a signal interconnection. To the contrary, a CM radiation amount or a major radiation amount including a CM radiation amount largely depends on the position of a signal interconnection. For this reason, the CM/DM characteristic or MAJ/DM characteristic is obtained as a coefficient that represents the position dependence of a CM radiation amount or major radiation amount. FIG. 20 shows a CM/DM characteristic when a 10-mm long signal interconnection was arranged at the center of a ground plane having a size of 210 mmxc3x97100 mm and a CM/DM characteristic when a signal interconnection was arranged at the central position with respect to the long side direction and at a position separated by 5 mm from a ground plane edge parallel to the signal interconnection. The results were obtained by electromagnetic field simulation in accordance with the same procedure as described above. As described in reference 2, as the distance between the signal interconnection and the ground plane edge decreases, the CM radiation amount increases in a broad band. However, even when the position of the signal interconnection changes, the peak frequency does not change. This means that when the shape of the ground plane is determined, the peak frequency of radiation by the ground plane is automatically determined.
FIG. 21 also shows a result obtained by electromagnetic field simulation. The result is obtained by calculating the relationship between the CM/DM characteristic and the position of a signal interconnection in the direction of ground plane width at the peak frequency of the CM/DM characteristic. As the distance between the signal interconnection and the ground plane edge decreases, the value of CM/DM exponentially increases.
The present inventor repeated similar measurements and calculations while changing the shape of the ground plane of a printed circuit board and the position of an interconnection and checked in detail the manner the CM/DM characteristic and MAJ/DM characteristic changed when the ground plane size or interconnection position was changed. The present inventor found that when the size of the ground plane was determined, the CM/DM characteristic and MAJ/DM characteristic could be given by simple equations on the basis of lengths a and b of the ground plane in the vertical and horizontal directions and a distance d between the signal interconnection and the ground plane edge. The present inventor also found that when these equations were used, the CM/DM characteristic and MAJ/DM characteristic at the position of a signal interconnection could be obtained independently of the size of a rectangular ground plane.
On the other hand, a DM radiation amount can be obtained by a simple theoretical equation when the shape of a signal interconnection and the value of a flowing current are determined, as is known.
Hence, when the CM/DM characteristic and MAJ/DM characteristic are obtained from the relationship between the ground plane shape and the signal interconnection position using the equations, and the DM radiation amount of the signal interconnection is separately obtained, a CM radiation amount generated from the ground plane in correspondence with the signal interconnection and the major radiation amount of entire electromagnetic radiation caused by the signal interconnection can easily be calculated in a short time on the basis of the CM/DM and MAJ/DM characteristics and the DM radiation amount. For example, it took about 48 hrs to obtain the calculation results of the CM/DM ratio and MAJ/DM ratio by electromagnetic field simulation shown in FIGS. 17 to 19 by using a personal computer with Pentium III (registered trademark of a CPU available from Intel) and a clock frequency of 1 GHz. However, the equations were used, the results could be obtained in 1 sec or less, though only the values at the peak frequency were obtained.
When a radiation amount is calculated on the basis of the above idea, the radiation amount from a printed circuit board, which depends on the position of a signal interconnection, can be grasped at the stage of board design. In addition, the radiation amount from a printed circuit board can be more accurately predicted as compared to a prior art which considers only DM radiation from a signal interconnection. Furthermore, a printed circuit board with suppressed electromagnetic radiation can be more accurately designed on the basis of the more accurately obtained radiation amount.